Required Practical: Infrared Absorption and Emission

How surfaces affect infrared energy transfer

Infrared radiation transfers energy from a source to an absorber. An object can emit infrared, transferring energy away from itself, and absorb infrared arriving from elsewhere, transferring energy to itself. Absorbing radiation can raise an object's temperature.

Warm objects emit infrared radiation. Hotter objects emit more, so a comparison of surfaces is only fair if their temperatures are the same. The nature of the surface also matters: a matt black surface is a good absorber and emitter of infrared, whereas a shiny silver surface is a poor absorber and emitter and a good reflector.

Colour and shininess are different properties. Matt black and shiny black surfaces have the same colour but different finishes. Comparing them helps investigate the effect of finish rather than treating colour as the only factor. Visible colour alone is not always a reliable guide to infrared behaviour: a white painted surface need not behave like shiny metal.

Comparing emission with a Leslie cube

A Leslie cube is a hollow metal cube with differently finished faces. The surfaces used here are shiny black, matt black, shiny silver and matt white. Filling the cube with hot water heats all four faces, allowing their infrared emission to be compared at approximately the same temperature.

The apparatus consists of a Leslie cube, a kettle, an infrared detector and a heatproof mat. A ruler is useful for setting a consistent detector distance.

1. Place the Leslie cube on the heatproof mat before filling it.
2. Carefully fill it with very hot water and replace the lid. Allow the faces to warm before taking readings.
3. Position the infrared detector facing the centre of one face. Wait for its reading to settle, then record the surface type and reading, including the unit shown by the instrument.
4. Measure each of the other faces with the detector at the same distance from the surface and facing it at the same angle.
5. Repeat the measurements so that you can check consistency and calculate a mean reading for each surface.

The independent variable is the surface type or finish. The dependent variable is the infrared detector reading, which indicates the radiation reaching the detector. Keep the detector distance, angle and settings unchanged. The equal-sized faces also allow equal surface areas to be compared.

Very hot water can scald, and the cube becomes hot. Filling should be supervised, the cube should remain on its mat, and students should not carry containers of hot water across the laboratory. Avoid touching the hot faces and keep water away from electrical equipment. Leave the apparatus to cool before handling or emptying it.

Presenting and evaluating the emission results

Record the readings in a table with one row for each surface. Draw a bar chart, with surface type on the horizontal axis and mean detector reading, with its unit, on the vertical axis. Surface types are separate categories, so they should have separate bars.

A higher reading means that more infrared reached the detector. With distance and temperature controlled, this indicates greater emission from that surface. The expected contrast is strong emission from matt black and weak emission from shiny silver.

For example, a set of Leslie-cube results gives infrared intensities of 19.5 W/m² for matt black, 14.2 W/m² for shiny black, 5.1 W/m² for matt white and 3.8 W/m² for shiny silver. In this example, matt black is the best emitter and shiny silver is the worst; shiny black emits more than matt white. Use your measured readings to order the surfaces rather than assuming that every material must follow a complete order based on its visible colour.

The cube cools during the investigation. If one face is always measured last, its lower reading might partly reflect cooling rather than its finish. Take the readings promptly and repeat them in a different order. For repeated sets, check that the water temperature is comparable.

An inconsistent detector position can also change the reading. Use a fixed distance measured with a ruler and aim at the centre of each face. Repeated readings help reveal random variation; however, repeating measurements does not correct a distance that is consistently wrong.

Comparing absorption with metal sheets

Absorption can be investigated by directing infrared at two similar metal sheets: one with a matt black surface facing the source and the other with a shiny metal surface facing it.

Use an infrared source such as a radiant heater, two stands and clamps, two small coins, candle wax and a stopwatch. Secure the sheets so that they face the source at equal distances. Attach a coin to the back of each sheet using a similar small amount of wax. The coins should be on the sides facing away from the source, so the source heats the sheets rather than directly heating the wax.

Start with both sheets at the same temperature. Switch on the source and time how long it takes for each coin to fall. Infrared absorbed at the front heats the metal; energy is transferred through the sheet to the wax. When the wax melts sufficiently, it can no longer hold the coin.

The coin on the matt black sheet is expected to fall first. Its shorter falling time indicates faster heating and supports the conclusion that matt black is the better absorber. The shiny surface reflects more of the incoming infrared and absorbs less.

For a fair comparison, use sheets of the same metal, size and thickness, identical coins, similar amounts of wax and equal source distances and angles. Here, surface finish is the independent variable and time until the coin falls is the dependent variable. Repeat only after the sheets have cooled and the coins have been reattached consistently.

The source and sheets become hot. Secure the clamps, keep combustible materials away from the heater, place a heatproof tray beneath the coins and do not touch the sheets or fallen coins until they have cooled. Switch off the source once the observations are complete.

Separating the effects of colour and finish

Temperature rise provides another way to compare absorption. In an investigation using four same-sized hollow metal cubes containing cold water, each cube was placed the same distance from an infrared heater. After 10 minutes, the temperature increases were 3.0°C for matt white, 2.0°C for shiny white, 6.5°C for matt black and 4.0°C for shiny black.

Compare like with like to draw two distinct conclusions. Matt black warmed more than matt white, and shiny black warmed more than shiny white: black surfaces absorbed more infrared than white surfaces in this investigation. Matt black warmed more than shiny black, and matt white warmed more than shiny white: matt surfaces absorbed more than shiny surfaces of the same colour. Matt black produced the greatest temperature rise and shiny white the smallest.

Here the dependent variable is the temperature increase after a fixed time, rather than a detector reading or a falling time. Equal water volumes, starting temperatures, cube dimensions, heater distances and heating times are needed for a fair comparison.

What the investigations show

The Leslie cube compares energy travelling from a surface to a detector. The absorption investigations compare energy travelling from a source into a surface. Together they demonstrate that surface finish affects both emission and absorption: good infrared absorbers are also good infrared emitters, while shiny metallic surfaces are poor at both.

Surface properties

• Matt black: good infrared absorber and emitter.
• Shiny silver: poor absorber and emitter; good reflector.
• Good infrared absorbers are also good emitters.
• Both colour and finish matter; visible colour alone is not always a reliable guide.

Emission: Leslie cube

• Fill the cube with hot water; measure infrared from shiny black, matt black, shiny silver and matt white faces.
• Change: surface type or finish. Measure: infrared detector reading.
• Control: surface temperature, detector distance and angle, detector settings and surface area.
• Repeat readings; vary measurement order to reduce the effect of cooling.
• Plot a bar chart of mean detector reading against surface type.
• Expected contrast: matt black gives a high reading; shiny silver gives a low reading.

Absorption: metal sheets

• Place matt black and shiny metal sheets equal distances from an infrared source.
• Attach identical coins to their backs with similar amounts of wax.
• Measure time until each coin falls. The matt-black sheet's coin is expected to fall first: greater absorption heats the sheet faster and melts the wax sooner.
• Keep sheet material, dimensions, initial temperature and source position the same.

Interpreting absorption results

• A greater temperature rise in a fixed time indicates greater absorption under controlled conditions.
• Compare the same finish to investigate colour and the same colour to investigate finish.
• In the cube comparison, black absorbed more than white, and matt absorbed more than shiny surfaces of the same colour.

Safety

Hot water, the cube, heater and sheets can burn or scald. Use heatproof protection, secure apparatus, avoid carrying hot water and allow equipment to cool before handling.